The present invention relates to a thermal overload relay for change-over of a contact upon detection of an overcurrent.
Patent Document 1, for example, discloses a thermal overload relay operated by detecting an overcurrent running in the main circuit.
The thermal overload relay of Patent Document 1 is described referring to FIGS. 8 and 9. As shown in FIG. 8, the thermal overload relay comprises, an insulator case 1 made of a resin mould which houses main bimetals 2 inserted in three phase electric circuit and wound with heaters 2a, a shifter 3 linked to free ends of the main bimetals 2 and movably supported in the insulator case 1, a reversing mechanism 4 disposed in the insulator case 1 linkable to one end of the shifter 3, and a switching mechanism 5 to changeover contacts by operation of the reversing mechanism 4.
The reversing mechanism 4 comprises, as also shown in FIG. 9, a temperature compensation bimetal 7 to link to the one end of the shifter 3, a release lever 8 to which the other end of the temperature compensation bimetal 7 is fixed, and an adjusting link 12 connecting to the release lever 8 through a swinging pin 9 projecting at the lower end of the adjusting link and abutting on the circumferential surface of an eccentric cam 11a. This cam 11a is associated with an adjusting dial 11 disposed rotatably in the insulator case 1 at the upper end of the adjusting link 12. A rotation angle of the release lever 8 is set by varying an abutting position of the adjusting link 12 with the circumferential surface of the eccentric cam 11a of the adjusting dial 11 through adjustment of the adjusting dial 11, thereby slightly rotating the adjusting link 12 around a support shaft 13.
The switching mechanism 5 comprises: a reversing spring 14 fixed at its lower end to the release lever 8 and extending upwards, a slider 17 linked to the tip of the reversing spring 14 and carrying a normally opened side movable contact piece 15b and a normally closed side movable contact piece 16a, and a reset bar 18 to manually move the slider 17 to the normal position. The switching mechanism 5 further comprises the above mentioned normally opened side movable contact piece 15b and the normally closed side movable contact piece 16a, and a normally opened side fixed contact piece 15a and a normally closed side fixed contact piece 16b. Both the fixed contact pieces are disposed opposing the movable contact pieces. The reversing spring 14 is a member having a punched window 14a formed by punching a thin spring material and a curved surface with a disc spring shape around the punched window 14a. The reversing spring 14 is curved with a convex towards right hand side in a normal state shown in FIG. 8.
When the bimetal 2 bends with the heat generated by the heater 2a due to an overcurrent in the above-described structure, the shifter 3 shifts to the direction indicated by the arrow P in FIG. 8 caused by displacement of the free end of the main bimetal 2. The shift of the shifter 3 pushes a free end of the temperature compensation bimetal 7 and rotates the release lever 8 counterclockwise around the swinging pin 9.
With the progression of the counterclockwise rotation of the release lever 8, the reversing spring 14 deforms, bending convexly towards the left hand side (as seen in FIG. 8). The deformation of the reversing spring 14 moves the slider 17 linked to the tip of the reversing spring 14 so as to turn the normally opened side movable contact piece 15b and the normally opened side fixed contact piece 15a into a closed state and to turn the normally closed side movable contact piece 16a and the normally closed side fixed contact piece 16b into an opened state. Based on the indication of the closed state of the normally opened side movable contact piece 15b and the normally opened side fixed contact piece 15a, and the information of the opened state of the normally closed side movable contact piece 16a and the normally closed side fixed contact piece 16b conducted by the reversing action of the reversing mechanism 4, an electromagnetic contactor (not shown in the figures), for example, connected in the main circuit is opened to interrupt the overcurrent.
[Patent Document 1]
Japanese Examined Patent Publication No. H7-001665
Meanwhile, in the conventional thermal overload relay described above, if the support shaft 13 of the switching mechanism 4 projecting out of the inner wall of the insulator case 1 is worn by prolonged use, a position of the pin 9, which is projecting out of the bottom of the adjusting link 12 and rotatably supporting the release lever 8, changes. The change of the position of the pin 9 induces change of position of the temperature compensation bimetal 7 fixed on the release lever 8.
Thus, the position change of the temperature compensation bimetal 7 due to wear of the support shaft 13 of the reversing mechanism 4 may cause a variation of a reversing operation point of the reversing mechanism 4 in the event of overload current. Therefore, the operation performance may be unstable in the thermal overload relay.
In view of the above-described unsolved problems in the conventional technology example, it is an object of the present invention to provide a thermal overload relay that suppresses variation of a reversing operation point of the contact reversing mechanism and performs stable operation of a thermal overload relay.
Further objects and advantages of the invention will be apparent from the following description of the invention.